A method for the grinding and cleaning of waste plastic, in particular mixed plastic, is known from WO 2006/100044, in which a compactate or an agglomerate is produced from film shavings or other film remainders and chopped plastic parts. The agglomerate drastically reduces the volume of the waste plastic and can thus be easily transported. In this state, it is largely used for power generation. A large portion of dirt, impurities and adherences thereby remain in the agglomerate/compactate. In the known process, knowledge assumes that such a compactate or agglomerate can be easily ground and is suitable for further processing and refining. The grinding preferably occurs in a disk or drum refiner in the presence of water. A fine-particle fraction is removed from the ground material emerging from the refiner. The remaining ground material is washed or mechanically dehydrated and dried. Through further processing, such ground material can be used as a replacement for wood in composite wood boards, as a filler material in various applications and, if the degree of purification is appropriate, even with pure plastic or high-quality sorted recycling plastics for the production of plastic parts. Another area of application is the production of so-called WPC parts (wood plastic composites). During the production of such parts, a mixture of wood and plastic particles is produced either through dry mixture and direct processing or through compounding with the help of an extruder, an agglomerator, a heating mixer or a heating-cooling mixer and processed into shaped parts.
Waste plastic naturally contains a considerable share of pulp that is unusable for many purposes. This pulp originates from adhesive labels, composite packaging or from incomplete separation from free paper. In the case of plastics from waste paper collections, larger amounts of pulp often stick to the plastics. During compacting, the pulp is melted or encapsulated and thereby ends up in the grinding process.
In the case of these materials under the term compactate, one must differentiate between an agglomerate, which comes from a disk, ring die or pot agglomerator, and pellets, which are produced via a sieve mold process. Both processes compact or agglomerate the material in a dry process and create a three-dimensional composite of plastic particles with a defined particle size.
The differences between the aforementioned agglomeration processes and the pelletization processes are that a partial or even complete melting of the plastic takes place in the agglomeration processes, while no melting or only slight melting on the edge takes place in the pelletization processes using sieve molds. Another difference is that a layering of the plastics in layers takes place in the sieve mold pellets, above all in the case of plastics from films, which is not the case with the agglomerates.
However, the most important difference can be found in the aforementioned dirt and adherences. In the case of agglomerates, a large portion of the adherences, in particular pulp and dirt, is melted. In the case of sieve mold pellets, there is partial encapsulation but no melting, since the material is not melted thoroughly.
Sieve mold pellets can only be ground in an unsatisfactory manner with the process described in WO 2006/100044, since the release of individual film flakes takes place easily, which are known in the process described there to be incompletely ground or not ground at all. Only when the three-dimensional deformation of individual film flakes in the sieve mold pellets is sufficiently strong is there a satisfactory grinding result. In the case of an agglomerate that is partially or thoroughly melted, there is always an excellent grinding result according to WO 2006/100044, wherein two-dimensional flakes that are not integrated into the agglomerate but are not melted are nonetheless not optimally ground. If plastic is processed together with pulp in an extruder or in an injection molding machine, the moisture of the pulp leads to steam formation, which makes processing difficult or even impossible. Pulp in ground plastic also has the disadvantage that it attracts moisture hygroscopically and holds it longer than in the plastic material during a drying process, if applicable.
The object of the invention is to specify a method for the recycling of waste plastic, in particular mixed plastic, with which the reuse possibility of the prepared waste plastic is improved and in particular the separation of adherences and/or pulp from the waste plastic is facilitated.